Data represents a significant asset for many entities. Consequently, data loss, whether accidental or caused by malicious activity, can be costly in terms of wasted manpower, loss of goodwill from customers, loss of time and potential legal liability. To ensure proper protection of data for business and legal purposes, many entities back up data to a physical storage media such as magnetic tapes or optical disks. Traditionally, backup would occur at each machine controlled by an entity. As the sophistication of network technology increased, many entities turned to enterprise level backup in which data from multiple machines on a network is backed up to a remote media library. Centralized data backup allows storage problems to be identified at one location and has the advantage of increased efficiency.
One example of a media library commonly used in enterprise backup systems is a magnetic tape library. In a typical magnetic tape library, tapes are contained in cartridges and the tape library contains multiple cartridge slots in which tape cartridges can be stored. The tape cartridges are physically moved between cartridge slots and tape drives by a robot. The robot is controlled by access commands received from the host devices on the network. When specific data is required, the host device determines which cartridge slot contains the tape cartridge that holds the desired data. The host device then transmits a move-element command to the robot and the robot moves the tape cartridge.
In a SCSI tape library, devices that are part of the library are typically addressed by target number and logical unit numbers (“LUN”). Thus, each drive and robot of a tape library typically has a target number and LUN. Cartridge slots, on the other hand, are addressed by element numbers that are used by the robot to locate the slots. Because the robot also places tape cartridges in the drives, each drive is also associated with an element number. If multiple tape libraries are connected to a single device (e.g., a fibre channel to SCSI router), the tape libraries may be further addressed by bus number.
Media libraries, such as tape and optical libraries, can range from small “auto loader” devices with a single media drive unit and a number of storage volumes to large libraries comprised of several dozen media drive units and hundreds, if not thousands of storage volumes. Some prior art media libraries include a single embedded controller to present the associated media library to other computers as a number of distinct smaller media libraries. These “partitioned libraries” may have some subset of the physical library's media drives, storage elements and import/export elements allocated to them, while sharing certain other physical library components such as media transport elements. Generally, the allocated elements have logical addresses assigned that must be mapped to actual physical addresses.
FIG. 1 illustrates one embodiment of a prior art media library system 100. System 100 includes multiple hosts 102a-102e connected via a storage area network (SAN) 105 and storage router 107 to media library 110. Media library 110 is typically a variation of a SCSI media changer, which generally contains at least one media changer 115 with an associated medium transport element 120 (alternatively referred to as a “robot” or “picker”), multiple storage elements 125 that can store storage volumes (e.g., tape cartridges, optical disks), and multiple data transport elements (drives) 130. Media library 110 can also have import/export elements 135. The media changer 115 is often a microprocessor based embedded controller that controls medium transport element 120. Medium transport element 120 physically moves storage volumes from one changer element to another. For example, medium transport element 120 can move a storage volume from a storage element to a data transport element in response to a “Move Medium” command.
In a partitioned library, a large, multi-storage element, multi-data transport element, multi-import/export element unit is represented to applications running on hosts (e.g., hosts 102a-102e) as a number of different smaller libraries, each with some allocated number of storage elements, some allocated number of data transport elements, and some number of allocated import/export elements. Each host may see a different media library that represents some subset of media library 110. Thus, for example, applications running on host 102a may see a different media library than other applications running on host 102a or applications running on host 102e. 
Access to the physical media library, in these prior art systems is typically provided by a single device, such storage router 107. Storage router 107 can maintain the media library partitions, as for example, as set of logical to physical mappings, and present the host applications with the appropriate media library partitions. Each host application can then form commands based on the media library partition presented to it. Storage router 107 can map the commands to the appropriate physical addresses and forward the commands to media library 110.
As the rates at which tape drives and other media drives can read/write data increases, providing access through a single storage router or other controller can be insufficient because the device providing access to the media library can act as a bottleneck, slowing the amount of data that can be written to a storage medium. Therefore, a need exists for a system that can provide access to a media library through multiple controllers.